Angular 9 support

[bsautel]

This pull requests add support to Angular 9.

To that, I first converted the project to Angular CLI with version 8 and then used Angular CLI to upgrade to Angular 9.

I added a DEVELOPER.md file that explains how to build and publish the library and the example app.

[bsautel]

Note that I added an end to end test to ensure the component is loaded and displayed when we scroll down.

I also published this version into a private repository and it works as expected in an Angular 9 project.

[annapogorelova]

Hey, @bsautel, thank you for your work and I am sorry that it took me so much time to get to this PR. Your changes look really good, but the problem is that there too many changes to the project structure. Looks like the Angular development team introduced this new template for the library projects, which is not quite compatible with my existing project. So, to make everything work with Angular 9, I decided to start a completely new git repository and a new NPM package, which I was going to rename to ng-go-top-button at some point of time anyway. I have already published it, you can find it here: ng-go-top-button. Thank you again for your work.

[bsautel]

Thanks @annapogorelova for creating this new package.

I just come to migrate to it instead a custom build I did from this pull request and it works as expected. I just had to change the component name, its prefix changed, but that's a good thing. 👍